The present invention relates to load-pulled oscillator circuits and systems.
DC Biasing circuits for oscillators typically incorporate radio frequency (RF) chokes (i.e. high inductance rf blocking inductors), with a shunt capacitance to ground immediately after the choke. This provides a high impedance to the RF while providing a DC path for the biasing of the active devices. (Providing a high RF impedance helps to avoid unwanted RF circuit paths which can cause unwanted oscillation modes, and also avoids damping of the RF oscillation.)
This conventional method encounters some difficulties in broadband RF circuits. At microwave frequencies almost any real-world passive device with have substantial reactance from series inductances and shunt capacitances, so that self-resonances can occur. For circuits which operate only over a limited frequency range this can usually be avoided by appropriate component selection, but for circuits which operate over a frequency range of an octave or more (i.e. whose highest required operating frequency is double the lowest) these resonances can be a problem. Even if the choke does not hit resonance as a single lumped element, partial resonances within the windings or leads can still cause frequency-dependent impedance changes, and this may cause the RF choke to be less than adequate isolation at some frequencies. Also, inductance-generated voltage spikes are detrimental to circuit and device performance.
One way to keep the choke""s impedance high over a range of frequencies would be to load the choke with ferrites or other magnetic materials. However, such materials typically have a large temperature coefficient, which can cause undesirable frequency shifts in oscillators. Even small lead or inter-winding resonances can affect the tuning characteristics of oscillators, causing power dips and (worse yet) backwards tuning (where frequency is NOT monotonically dependent on tuning voltage in a varactor tuned oscillator).
Load-pulled oscillators are an important technique for RF monitoring. A free-running oscillator, typically at VHF or higher frequencies, is electromagnetically coupled to some environment which is desired to be characterized. (For example, an unknown oil/water/gas composition can be flowed through a coaxial probe section.) Since the oscillator is not isolated from the environment being measured, changes in that environment will pull the frequency of oscillation. By monitoring shifts in the frequency of oscillation, changes in the environment being monitored can be seen with great precision. (For example, in compositional monitoring of wellhead flows of oil/gas/water mixtures, the environment being monitored is a medium having a variable composition, and changes in the composition are seen as shifts in the oscillation frequency for a given tuning voltage.) Such circuits are described e.g. in published PCT application WO 91/08469 (published 13 Jun. 1991), which is hereby incorporated by reference.
The load pulled oscillator requires a topology which will support oscillations throughout a change in the load impedance from the capacitive to inductive loading. Typically such an oscillator must cover octave or more bandwidths and remain capable of oscillating into any load impedance or phase within its range.
In order to design a system to be capable of this range, biasing and impedance effects of the associated DC networks must be considered from an RF or microwave perspective. DC biasing achieved from the use of RF chokes and resistors is effective only if the chokes are effectively an open with no resonances, and if the combinations of inductance, resistance and capacitance don""t limit the ability of the circuit to respond broad band.
Use of standard commercial grade inductors as RF chokes in the frequency range of 100 MHz to 1 GHz was accomplished by use of 100 nH to 2200 microH inductors of the coil variety. The problems were that for high frequency oscillators, the lower inductance allowed undesired low frequency oscillations to occur at certain loads and frequencies. The larger inductors supplied excellent blocking, but created problems at the second harmonic of the frequency, thus causing backward tuning and decrease in load pull sensitivities. These same problems can be even worse at higher microwave frequencies. These problems relate to self-resonance characteristics of the RF choke.
The need for broadband chokes in load pulled oscillators is particularly severe, since any change in frequency or impact on the oscillator performance limits the ability to resolve microwave parameters. Ideally, when such systems are used for measurement of material parameters, the oscillator frequency will respond very precisely to changes in the load impedance; but when components of the circuit are also changing their impedance as the oscillator frequency shifts, the relation between oscillator frequency and load impedance becomes less direct.
Improved RF Choke Bias Scheme for Load Pulled Oscillators
Load-pulled oscillators differ fundamentally from other RF circuits, in that the load-pulled oscillator is NOT required to be insensitive to variations in the circuit stages which follow it: the whole purpose of the load-pulled oscillator circuit is to respond to changes in load impedance, so isolation and similar techniques are not applicable. The present inventor has realized that in load-pulled-oscillators, unlike other RF oscillators, temperature dependence and impedance changes in the blocking chokes are important problems. The present application therefore teaches a new way to solve this problem: the innovative circuits combine a load-pulled oscillator with a blocking stage which uses a self-biased active device to emulate an RF choke.
The use of an active choke-emulating stage provides dynamic control of performance while creating an open circuit without parasitic resonances.
The disclosed innovations, in various embodiments, provide one or more of at least the following advantages:
monotonic tuning;
reduced temperature dependence;
reduced risk of unwanted frequency hop; and
reduced risk of spectral breakup.